1. Field of the Invention
The present invention relates to a Heterojunction Bipolar Transistor hereinafter also referred to as an “HBT”.
2. Description of Related Art
A heterojunction bipolar transistor of III-V compound semiconductors is used for communications such as optical communications, microwave or millimeter wave communications and also comes into practical use in, for example, high-frequency or high-power elements of mobile communication devices or optical communication devices since such heterojunction bipolar transistor shows better properties in high-frequency and higher current drive ability than a bipolar transistor being composed of a single material system.
For putting the HBT into practical use, it is especially important to improve its reliability. Though many approaches have been made for improving the reliability of the HBT, the improvement in the reliability of the HBT is still unsatisfactory since a device sometimes degrades due to the HBT therein if a large current like that used for a power device is applied to the HBT.
Major cause of such degradation is considered as follows. C (carbon), which is generally used as a p-type dopant in place of Be in recent years, doped in a GaAs base layer of an AlGaAs/GaAs HBT has a smaller atomic radius than those of Ga and As of the base layer. Therefore, C strains the base layer and functions as a recombination center resulting in a reduced current gain and a lowered reliability especially in case of high doping of C.
As a way to prevent this problem, it is proposed to add elements of column III or V other than those composing the base layer to the C-doped GaAs base layer. See, for example, Japanese Patent Kokai Publication No. H6-37105. According to this way, the addition of impurities as dopants (e.g. In or Sb) having a larger atomic radius than those of the elements composing the base layer (i.e. Ga and As) can alleviate strain of the base layer to improve reliability of HBT.
As another way, it is also proposed to use two types of impurities composed of elements excepting column III or V as dopants of the GaAs base layer. One of two types of impurities has a larger atomic radius, and the other has a smaller atomic radius compared with the atoms composing the base layer. See, for example, Japanese Patent Kokai Publication No. 2000-323491. According to this way, the addition of the one type of impurities having a larger atomic radius (e.g. Mg) and the other type of impurities having a smaller atomic radius (e.g. C) compared with the atoms composing the base layer (i.e. Ga and As) can alleviate strain of the base layer without forming compounds with Ga or As to improve reliability of HBT.
An example of the HBT according to the latter way in prior art will be described below with reference to the accompanied drawings. (See an embodiment of FIG. 1 in Japanese Patent Kokai Publication No. 2000-323491.) As shown in FIG. 7, a conventional HBT 600 is produced by forming on a semi-insulating GaAs substrate 601, an i-GaAs or i-AlGaAs buffer layer 602, an Si-doped n-GaAs collector layer 603, a C and Mg-doped p-GaAs base layer 604, an Si doped n-AlGaAs or n-InGaP emitter layer 605, and an Si-highly doped n+-GaAs emitter-cap layer 606 in order by using the technique of epitaxial growth. Then, the n+-GaAs emitter-cap layer 606, the n-AlGaAs or n-InGaP emitter layer 605 and the p-GaAs base layer 604 are dry etched appropriately, and a WSi emitter electrode 609, an Ni/AuGe/Au collector electrode 607 and a Ti/Pt/Au base electrode 608 are formed on the n+-GaAs emitter-cap layer 606, the n-GaAs collector layer 603 and the p-GaAs base layer 604 respectively. The HBT 600 is called as an AlGaAs/GaAs HBT when a material of the emitter layer is n-AlGaAs, or as an InGaP/GaAs HBT when a material of the emitter layer is n-lnGaP. It is known that the InGaP/GaAs HBT have a longer lifetime and a higher reliability compared with the AlGaAs/GaAs HBT.
In order to dope new element such as In, Sb or Mg in addition to C which is conventionally used as dopants, however, an existing apparatus utilizing the technique of epitaxial growth could not be applied. It becomes necessary to alter the existing apparatus at the minimum otherwise to construct a new apparatus.
Furthermore, the base layer doped with element such as In, Sb or Mg in addition to C has a dry etching rate which is extremely lower than that of the base layer without the element such as In, Sb or Mg. In order to etching the former base layer, it is necessary to supply a gas(es) for dry etching which is different from those conventionally used, or to conduct ion milling. When conducting the ion milling, an etching mask must be formed to have a larger film thickness since the etching mask is etched at an amount almost same as the base layer. (In other words, it is difficult to achieve high selectivity of etching). However, it is difficult to form the etching mask having a large thickness with a fine pattern on an uneven layer. In any case, it is required to alter the existing apparatus and steps for producing the HBT using the technique of epitaxial growth.
The present invention aims to provide an HBT having a high reliability without extensively altering the existing apparatus and steps for producing it.